1. Field of the Invention
The present invention relates generally to facilities for separating oil and water present in a fluid mixture produced from a subterranean well. In another aspect, the invention concerns a system for controlling an oil field separation facility by measuring the amount of oil present in the separated water stream.
2. Description of the Prior Art
Oil wells frequently produce water along with oil. In many instances the commonly produced oil and water phases are separated in the field prior to transporting the oil to a major pipeline or refinery facility. Each in-the-field oil separation facility may service a number of individual wells. Thus, large volumes of oil and water may be passed through the separation facility every day. If the separation facility does not properly separate the oil and water, either the oil stream exiting the facility will contain too much water or the water stream exiting the facility will contain too much oil. If too much water is contained in the separated oil stream exiting the facility, the oil stream may be rejected by the operator of the pipeline and/or the refinery. If too much oil is contained in the separated water stream exiting the facility, the value of the oil present in the water stream is lost because typically the separated water stream is simply discharged or reinjected into the subterranean formation for water flooding. For large separation facilities, an oil content of just 5,000 ppmw in the separated water stream can result in a yearly oil loss valued at several million dollars.
In the past, the oil content of the separated water stream from oil field separation facilities has been measured on a relatively infrequent (typically daily) basis. However, it has recently been discovered that the amount of oil in the separated water stream can vary greatly by the hour or minute. Thus, measuring the oil content of the separated water stream on an infrequent basis may not provide a reliable indication of oil content because limited duration spikes in the oil content may not be accounted for. Also, infrequent measurement of the oil content does not allow the separation facility to be controlled in a dynamic manner which can reduce or eliminate spikes in the oil concentration. An additional drawback of many conventional methods for measuring the oil content of a separate water stream is that past methods of sampling the separated water stream are somewhat suspect because they did not account for the tendency of the oil and water phases to separate from one another. Thus, conventional sampling methods drawing from a certain location in the separated water conduit may provide a poor representation of the actual content of oil in the separated water stream.
In the past, ultraviolet or visible fluorescence analyzers have been used to measure the oil content of diluted water discharged from separation facilities. These conventional analyzers are designed to measure much lower concentrations of oil (e.g., 10-50 ppmw) than are present in the undiluted separated water stream from the separation facility. Thus, because these conventional analyzers are not designed for the application proposed herein, they present a number of drawbacks. For example, these conventional analyzers are extremely sensitive to the amount of oil in the separated water stream and, therefore, would require high amounts of dilution (e.g., 100-10,000×) of the separated water sample in order to reduce the oil content of the analyzed sample down to a measurable level. Such high dilution rates result in a less accurate measurement due in part to trace amounts of organic contaminants present in the diluent. In addition, these conventional analyzers are designed for single phase fluids, rather than the 2-phase oil-in-water dispersions that are typical in the separated water stream. It is even more difficult for these conventional analyzers to provide an accurate measurement from a highly diluted sample because of the difficulty involved in providing uniform dispersions of oil in highly diluted water samples. Further, conventional analyzers must be recalibrated every time the composition of the oil in the separated water stream changes because conventional analyzers only measure a certain narrow range of oil compositions.